The clarity of the prior art calendars is insufficient, and it is difficult for the user to quickly establish an idea of the tasks required in a longer period and marked in the calendar. To obtain a general view, shifts are often required between various displays, including either month- or day-specific displays and calendar views, which makes the operation slower and further confuses the general impression. Furthermore, portable electronic devices often have the problem of limited size and resolution of the display, wherein the quantity of text-format data fitting on the display is limited, and shifting between various calendar views is increased. Systems of the prior art have the advantage of the capacity to store large quantities of data but at the cost of clarity, and they do not provide any means for the monitoring and evaluation of, for example, the month-specific workload on an overall level, because the systems are even too detailed in that respect.